The invention relates to a process for producing polyethylene having a bimodal and/or broad molecular weight distribution in a multi-stage process.
In certain applications, where films, bottles, cables and pipes are produced by extrusion or blow moulding, the polyethylenes having a narrow molecular weight distribution are not satisfactory because of their low melt flow properties and poor processability. Therefore different approaches have been suggested for manufacturing polyethylenes having a broad molecular weight distribution.
One approach to widen the molecular weight distribution is to blend a low molecular weight ethylene polymer with a high molecular weight ethylene polymer either mechanically or in solution. However according to this method it is very difficult to achieve sufficient homogeneity and/or expensive equipment is necessary for solution mixing, which makes these methods uneconomical and unpractical.
Attempts have been made to broaden the molecular weight distribution by a proper selection of catalysts. However the degree of broadening the molecular weight by this way is rather small. Also the activity of the catalyst tends to decrease quickly and therefore it is necessary to remove the catalyst residuals by washing.
There are also known various multi-stage processes for broadening the molecular weight distribution by carrying out the polymerization using different hydrogen concentration in each stages. This can be achieved either by polymerizing at a high hydrogen concentration in the first stage and at a low hydrogen concentration in the second phase, or vice versa. In the former case it is necessary to remove the unreacted gases and hydrogen after the first stage. In the latter case the conventional Ziegler-Natta catalysts tend to lose their activity during the progress of polymerization already at the first stage. The rate of polymerization, which is initially high, decreases at the second stage reactor because of the lowered activity of catalyst and high hydrogen concentration. As a consequence the residence time in the second stage becomes much longer than in the first stage. This means larger size of reactor at the second stage and difficulties in the control of the whole process.
Different polymerization methods can be used in multistep processes. Known multistep processes are for example slurry-slurry processes, gas phase-gas-phase processes or slurry-gas phase processes. As examples of slurry-slurry processes can be mentioned U.S. Pat. No. 3,592,880, EP 057 420, EP 237 294, GB 2 020 672, U.S. Pat No. 4,352,915 and EP 057 352. As examples of gas phase-gas phase processes GB 1 505 017, EP 040 992 and U.S. Pat. No. 4,420,592 can be mentioned. As examples of slurry-gas phase processes GB 1 532 231, U.S. Pat. Nos. 4,368,291, 4,309,521, 4,368,304 and EP 283 512 can be mentioned.
The present invention relates to a multi-stage process, in which a slurry-gas phase process is used. Therefore the last mentioned group of patent publications is reviewed here in more detail to clarify the state of the art in this field.
GB 1 532 231 discloses a two-step liquid-gas phase process, where in the first stage an olefine is polymerized in liquid monomer. The liquid phase is partially separated from the polymer and the resulting concentrated mixture is transferred to the gas-phase reactor. Because in the slurry phase according to this publication the polymerization is carried out in liquid monomer, it is evident that this process cannot be used for ethylene polymerization. Another disadvantage in this process is the formation of a solution of low molecular weight polymer in liquid monomer which causes problems related to product stream handling.
U.S. Pat. No. 4,368,291 discloses a two-step liquid-gas phase process, in which an olefine is polymerized in liquid hydrocarbon medium in a stirred tank type reactor. After the first polymerization step the mixture containing polymer particles and liquid hydrocarbon is as a whole transferred to the gas-phase reactor. One disadvantage of this process is that a great amount of hydrocarbon medium used in the first step is transferred to the gas-phase reactor disturbing polymerizing conditions there. The molecular weight control is also difficult in a conventional stirred tank reactor. Separation of heavier diluent is also more difficult and uneconomical.
U.S. Pat. Nos. 4,309,521 and 4,368,304 concern special catalysts, which can be used in liquid-gas-phase processes. These publications do not afford any useful information about the processes as itself.
Lastly EP 283 512 concerns a multi-stage process using certain specified catalyst. According to this publication liquid propylene or other liquid olefine is first prepolymerized in a liquid-phase reactor, for example in a loop reactor, the residence time being from 10 seconds to 400 seconds, after which the polymerization mixture as a whole is transferred to a gas-phase reactor, where polymerization is continued in gas phase. This process is thereby similar to GB 1 532 231 mentioned above and the process cannot be used in ethylene polymerization. The loop reactor step is merely a prepolymerization step. This patent publication is referred to here only because a loop reactor is mentioned in connection with a gas-phase reactor.